Epidemiologic studies suggest a lowered risk of hormone-dependent cancers among vegetarians. Vegetable and fruits contain lignans and isoflavones which can be converted to biologically active hormone-like substances by intestinal flora. The interaction of these compounds with endogenous hormones has not been evaluated and may be an important mechanism in cancer prevention. We have established an assay system that identifies estrogenic factors in the diet. We used MCF-7 human breast cancer cells since it is known that the transcription of the pS2 gene is directly controlled by the action of estradiol in this estrogen receptor positive cell line. The expression of pS2 RNA was monitored by Northern blot analysis using a non- radioactive DIG-labeled probe. The effect of various phytoestrogens including enterolactone, enterodiol, equol, nordihydroguiaretic acid (NDGA), genistein, kaempferol, daidzein and quercetin on pS2 expression in MCF-7 cells were studied. From our results, it appeared that equol, genistein, daidzein and kaempferol are able to elicit a strong pS2 response; enterolactone evokes a milder response while quercetin and enterodiol are inactive. The effect of these different compounds on cell growth corroborated their estrogenic effects on pS2 expression. The ability of enterolactone, quercetin, genistein, NDGA and equol to compete with estradiol for binding to the estrogen receptor was also evaluated. Genistein, equol, and NDGA competed with estadiol for binding to its receptor while enterolactone and quercetin did not. We have devised a sensitive assay to assay extracts of fruits and vegetables as well as to test various diet-derived components for estrogenic activity. Furthermore, we are planning to study the effect of various compounds on other cell lines derived from tissue other than breast. In addition, we will emphasize the mechanism of action of lignans and flavonoids at the molecular level.